Steel plate and steel plate coil

ABSTRACT

In a steel plate having tensile strength of 1 GPa or more at the normal temperature, the steel plate is made, stress decrement (SD) after uniform elongation in the stress-strain diagram obtained by using a tabular specimen of which is not less than 1.8×10 2  MPa, yield ratio (YR) of which is preferably not lower than 0.7, further preferably not lower than 0.8, and matrix structure of which is composed of martensite or bainite, or mixture of martensite and bainite substantially without proeutectoid ferrite, whereby the tough-ductility is improved on a practically utilizable level, and the steel plate becomes possible to be supplied in large quantities by winding up into a coil.

FIELD OF THE INVENTION

The present invention relates to a high-utilizable high-strength steel plate having tensile strength of 1 GPa or above, a steel plate coil obtained by winding up the steel plate of this kind and a method for producing such the steel plate and the steel plate coil.

DESCRIPTION OF THE RELATED ART

A high-strength steel plate with tensile strength of several hundreds of MPa is suggested (for example, refer to patent documents 1 and 2).

Until now, workability of the steel plate has been considered to be excellent in a steel well-balanced between strength and elongation, and research & development have been continued with a target for increasing total elongation in a case of steel excellent in the tensile strength.

Patent document 1: JP, 06-35619,B Patent document 2: JP, 2005-97725,A

However, it has been impossible to obtain the steel plate with practical tough-ductility (property combined with ductility and toughness) and tensile strength of 1 GPa or more at the ordinary temperature up to the present.

In general, the steel plate is supplied to the succeeding process as a hot-rolled coil, and put to practical application after cold working, cold rolling, and further heat treatment such as annealing or so.

However, in the high-strength steel plate of this kind, a considerable amount of cost and time are expended in process until the rolling, and there is scatter in the strength and yield rate is also inferior. Accordingly, it is the present situation that the high-strength steel plate is not in a stage of mass application even now.

SUMMARY OF THE INVENTION

The present invention is made in view of the aforementioned situation of the conventional high-strength steel plate, and it is an object of this invention to provide a steel plate having tough-ductility for practical use and possible to be supplied in large quantities as a steel plate coil and a method for producing the same.

The present inventors, as a result of repeating assiduous studies for achieving the above object, took note of stress decrement (SD) after the uniform elongation in a stress-strain diagram obtained by the tensile test using tabular specimens cut out from steel plates, and found the object to be accomplished by controlling the SD so as not to be less than predetermined value or more.

The present invention is based on the above-mentioned findings, the steel plate according to this invention is a steel plate having the tensile strength not lower than 1 GPa at the normal temperature, and characterized in that the stress decrement (SD) after uniform elongation in a stress-strain diagram obtained by a tensile test using a tabular specimen is not lower than 1.8×10² MPa. Preferably, the steel plate is characterized by having a yield ratio (YR) of 0.7 or above, more preferably 0.8 or above, and further characterized by having matrix structure composed of martensite or bainite, or mixture of martensite and bainite, and not substantially containing proeutectoid ferrite.

In the method for producing the steel plate according to this invention, the steel plate is rolled with minimum strain rate of not lower than 1×10/s at the same time of maintaining the steel plate in an austenitic phase, and quenched to a temperature lower than Ar1 point and not lower than Ms point, and preferably quenched to a temperature lower than Ar1 point and not lower than Ms point at average cooling rate of 1×10° C./s or more. Furthermore, the method is characterized in that obtained steel plate is subjected to tempering or annealing at a temperature of Ac1 point or below.

The steel plate coil according to this invention is made by winding up the aforementioned steel plate. In the method for producing the steel plate coil according to this invention, the steel plate is rolled with minimum strain rate of not lower than 1×10/s at the same time of maintaining the steel plate in an austenitic phase, and quenched to a temperature lower than Ar1 point and not lower than Ms point just before winding, and preferably quenched to a temperature lower than Ar1 point and not lower than Ms point at average cooling rate of 1×10° C./s or more from the exit of rolling. Furthermore, the method is characterized in that obtained steel plate coil is subjected to tempering or annealing at a temperature of Ac1 point or below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing a stress-strain diagram obtained by using tabular specimen.

FIG. 2 is a perspective schematic view illustrating rolling process of the steel plate coil in examples.

FIG. 3 is a view illustrating cutting points of tensile specimens for examining tensile strength variation in the cross direction of the steel plate coil.

FIG. 4 (a)˜(c) are magnified photographs showing fractured portions of tensile specimens according to examples 1, 6 and 16, respectively.

FIGS. 5 (d) and (e) are magnified photographs showing fractured portions of tensile specimens according to comparative examples 1 and 6 respectively, and FIG. 5 (f) is an illustration showing camera angle of these photographs.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, the steel plate and steel plate coil according to this invention will be explained in detail together with a production method thereof.

FIG. 1 is a schematic diagram showing a stress-strain diagram, which is obtained by using tabular specimen (for example, No. 5 specimen and No. 13 specimen specified in JIS Z 2201) cut out from the steel plate according to this invention. In this diagram, stress decrement (SD) is defined as the difference between tensile strength (TS) and rapture stress.

In the steel plate according to this invention, the stress decrement (SD) until the specimen is fractured after indicating uniform elongation in the above-mentioned stress-strain diagram takes value of 1.8×10² MPa or more.

The steel plate according to this invention is excellent in the tough-ductility (to be excellent in ductility and toughness at the same time) as compared with high-strength TRIP (Transformation Induced Plasticity) steel, as the SD value of the steel plate is not lower than 1.8×10² MPa.

Further, it becomes possible to wind up the steel plate into a coil in the final process of the steel making, and becomes possible to recoil (to roll again steel plate coiled once, concretely skin pass, cold rolling and the like, for example) afterward, therefore providability of the steel plate is remarkably improved, and it becomes possible to supply the steel plates in large quantities.

Although it is not always clear as to a reason for obtaining such the effects in this invention, the SD value indicating stress decrement after the uniform elongation of the steel plate according to this invention is much better than that of conventional steels, so that it seems that the aforementioned SD value has a considerable influence on the tough-ductility in the steel plate having tensile strength of 1 GPa or above.

In the conventional high-strength steel plates, much importance has been attached to formability of plate materials having TS (tensile strength) of 1 GPa or below, and special attention has been paid to improve the uniform elongation by strenuously reducing yield ratio YR=YS (yield strength: 0.2% yield stress or lower yield point)/TS to a value lower than 0.7. However, it is possible to increase the yield ratio YR to 0.7 or more in the steel plate having TS of 1 GPa or above in the steel plate according to this invention, whereby it becomes possible to improve proof stress, impact absorbency and so on of the steel plate.

As a method for obtaining the steel plate of this invention which is excellent in the tough-ductility by increasing SD value and YR according to grain refining and fine dispersion of the second phase particles such as carbides, a method is exemplified for rolling the steel plate with predetermined strain rate of not lower than 1×10/s, for example, at the same time of maintaining the steel plate at a temperature within an austenite range, and quenching the steel plate to a temperature lower than Ar1 point and not lower than Ms point at average cooling rate of 1×10° C./s or more desirably, thereby making matrix structure of the steel plate so as to be composed of martensite and/or bainite substantially without containing proeutectoid ferrite.

The steel plate of this kind is desirable to be wound into the form of steel plate coil, whereby it becomes possible to supply the high strength steel plates in large quantities, and enable the application in large quantities for various purposes.

Furthermore, such the steel plate coil is preferable to be manufactured by winding up the steel plate immediately after rolling, and possible to be manufactured by rolling with strain rate of not lower than 1×10/s at the same time of maintaining the steel plate in the austenite range, and quenching the steel plate to a temperature lower than Ar1 point and not lower than Ms point just before the winding.

In the case of quenching the steel plate to the temperature lower than Ar1 point and not lower than Ms point just before the winding, it is desirable to carry out the quenching at an average cooling rate of 1×10° C./s or above. In such the manner, it is possible to obtain the high strength coiled steel plate without dispersion of the strength in the cross direction, and possible to further facilitate the application of the steel plate.

It is preferable to further subject the steel plate and the steel plate coil obtained by the aforementioned method to tempering or annealing at a temperature of Ac1 point or below, thereby improving the aforementioned properties and increase stability of these properties.

It is necessary to pay attention not only to the manufacturing condition but also to chemical compositions of the steel in order to make the proeutectoid ferrite disappear and obtain the matrix structure merely composed of martensite and/or bainite with more certainty.

In this invention, it is certified that the directing steel is easy to be obtained by rolling the steel containing not lower than 0.1% of C, not lower than 0.15% of Mo, not lower than 0.5% of Cr, not lower than 0.3% of Ni under the aforementioned conditions, as a result of observing the difference of effects produced by the rolling conditions concerning many steels having various chemical compositions.

EXAMPLES

Hereinafter, this invention is further explained in detail on basis of examples, but this invention is not limited to these examples.

In the inventive examples and comparative examples described below, it is necessary to allow for errors of at least 10° C. above and below in the measurement of temperatures of every kind at the time of rolling, therefore it is proper to indicate the respective temperature as a temperature zone of 25° C. in view of actual condition in the measurement of temperatures. Also concerning the measured value of stress in the tensile test, it is suitable to consider the measured value of not higher than 10 MPa to be in the error range according to accuracy of specimen and other measuring conditions.

In the finish rolling process as shown in FIG. 2, rolled coils and steel plates were manufactured by rolling 12 kinds of steel ingots having chemical compositions (mass %) shown in Table 1 under rolling conditions shown in Table 2.

At the time of manufacturing the steel plates, the rolling was carried out by a rolling simulator of 1 stand general type. In the Table 1, the respective steels contain Si, Mn, P, S and Cu within ordinal ranges in addition to compositions described in the table.

TABLE 1 Chemical Compositions (%) Steel C Ni Cr Mo Fe A 0.38 1.02 1.94 0.65 bal. B 0.19 1.00 1.98 0.62 bal. C 0.40 2.01 2.00 1.03 bal. D 0.40 2.01 2.00 1.01 bal. E 0.21 2.02 2.01 1.01 bal. F 0.22 2.00 2.00 1.00 bal. G 0.34 0.09 1.03 0.17 bal. H 0.29 1.00 1.98 0.61 bal. I 0.22 — 2.02 1.02 bal. J 0.39 1.00 2.03 1.03 bal. K 0.39 0.99 2.02 0.51 bal. L 0.40 1.00 2.00 0.70 bal.

The combination of respective steels and rolling conditions is shown in Table 2 together with measured values of minimum strain rates at the time of finish rolling, respective temperatures at the inlet (start) and the outlet (end) of rolling, temperatures at the position just before the winding and average cooling rates.

TABLE 2 Minimum Temp. just Starting Test strain Temp. at Temp. at Cooling rate (×10° C./s) before Temp. of piece rate inlet outlet from outlet from outlet winding cooling Tempering No. Steel Form (/s) (×25° C.) (×25° C.) to winding down to 500° C. (×25° C.) (×25° C.) condition Remarks 1 A coil 10 42 32 4 — 21 — — Comp. ex. 1 2 A coil 6 42 33 5 — 21 — — Comp. ex. 2 3 A coil 6 42 33 5 — 20 — — Comp. ex. 3 4 A coil 6 42 36 5 — 22 — — Comp. ex. 4 5 C plate 9 36 26 — 0.4 — 20 — Comp. ex. 5 6 D plate 9 36 27 — 0.4 — 20 — Comp. ex. 6 7 E plate 9 36 27 — 0.4 — 20 — Comp. ex. 7 8 F plate 9 36 27 — 0.4 — 20 — Comp. ex. 8 9 A coil 20 42 37 9 — 14 — Example 1 10 A coil 20 42 37 10 — 13 — Example 2 11 A coil 20 42 37 10 — 13 500° C. × 1 h Example 3 12 A coil 20 42 37 10 — 9 500° C. × 1 h Example 4 13 A coil 20 42 37 10 — 13 600° C. × 1 h Example 5 14 A coil 20 42 37 10 — 9 600° C. × 1 h Example 6 15 B coil 20 42 37 9 — 14 — Example 7 16 B coil 20 42 37 9 — 10 — Example 8 17 B coil 20 42 37 9 — 14 500° C. × 1 h Example 9 18 B coil 20 42 37 9 — 14 600° C. × 1 h Example 10 19 G coil 20 42 32 6 — 16 — Example 11 20 C plate 10 44 30 — 1 — 19 — Example 12 21 D plate 10 44 33 — 1 — 19 — Example 13 22 E plate 10 44 32 — 1 — 19 — Example 14 23 F plate 10 44 33 — 1 — 19 — Example 15 24 H plate 20 36 28 — 1 — 14 — Example 16 25 I plate 10 32 25 — 1 — 19 — Example 17 26 I plate 10 36 27 — 1 — 19 — Example 18 27 J plate 30 35 34 — 1 — 16 — Example 19 28 K plate 30 35 33 — 1 — 16 — Example 20 29 L plate 30 35 33 — 1 — 16 — Example 21 30 B coil 20 43 38 6 — 22 — — Example 22 31 B coil 20 43 39 6 — 23 — — Example 23 32 B coil 20 43 38 6 — 22 — 550° C. × 1 h Example 24 33 B coil 20 43 39 6 — 23 — 550° C. × 1 h Example 25

In the examples of this invention, the rolled coils were manufactured by cooling at average cooling rate not lower than 1×10° C./s down to temperature just before the winding from outlet temperature so as to wind up the rolled steel plates at the temperature lower than Ar1 point and not lower than Ms point.

Further, in examples (inventive examples) 3 to 6, 9, 10, 24 and 25, obtained rolled coils were subjected to tempering at 500° C.˜600° C. (not higher than Ac1 point) for 1 hour.

On the other side, in examples (inventive examples) 12 to 18, and comparative examples 5 to 8, steel materials with initial thickness of 17 mm were rolled down to 3 mm through 5 passes in the austenite range. In the examples 12 to 18, the steel plates were manufactured by cooling them slowly down to the room temperature after quenching them dawn to the temperature lower than Ar1 point and not lower than Ms point at cooling rate of not lower than 1×10° C./s. In examples (inventive examples) 19 to 21, steel materials with initial thickness of 3.5 mm were rolled down to 1.9 mm through 1 pass in the austenite range, and the steel plates were manufactured by cooling them slowly down to the room temperature after quenching them dawn to the temperature lower than Ar1 point and not lower than Ms point at cooling rate of not lower than 1×10° C./s.

The rolled coils and the steel plates manufactured under the condition shown in Table 2 did not substantially contain proeutectoid ferrite, and matrix structures of them were composed of martensite or bainite, or mixture of martensite and bainite.

No. 5 specimens (25 mm width) specified in JIS Z 2201 were cut out from respective steel plate coils manufactured by the aforementioned procedure, and No. 13B specimens (12.5 mm width) specified similarly in JIS Z 2201 were cut out from respective steel plate, from the center of width along the rolling direction of respective plates. By using these specimens, tensile tests were carried out in conformity to JIS Z 2241, thereby obtaining stress-strain diagrams as shown in FIG. 1. Furthermore, yield strength (YS: 0.2% proof stress or lower yield point), tensile strength (TS) and stress decrement (SD) after uniform elongation were obtained from the stress-strain diagram, respectively, and yield ratio (YR) was calculated.

In order to further ascertain the level of dispersion of the tensile strength, tensile specimens were cut out from the respective positions of ½ W, ¼ W and ⅛ W (W: width of rolled coil) along the rolling direction as shown in FIG. 3, and tensile tests were carried out by using these specimens. In this time, dispersion of the tensile strength is given as a difference between maximum and minimum values of obtained tensile strength. These characteristic values are arranged respectively as shown in Table 3.

TABLE 3 Test Results of tensile test piece Thickness Specimen Tempering TS SD Tough- Dispersion of No. Steel Form (mm) JIS condition (GPa) YR (×10² MPa) ductility TS (MPa) Remarks 1 A coil 1.9 No. 5 — 1.7 0.6 0.3 No good 87 Comp. ex. 1 2 A coil 1.9 No. 5 — 1.7 0.6 0.6 No good 210 Comp. ex. 2 3 A coil 1.9 No. 5 — 1.6 0.6 0.3 No good 64 Comp. ex. 3 4 A coil 1.9 No. 5 — 1.5 0.6 0.7 No good 126 Comp. ex. 4 5 C plate 2.0 No. 13B — 1.5 0.7 1.7 No good — Comp. ex. 5 6 D plate 2.0 No. 13B — 1.6 0.7 1.2 No good — Comp. ex. 6 7 E plate 2.0 No. 13B — 1.3 0.6 1.7 No good — Comp. ex. 7 8 F plate 2.0 No. 13B — 1.4 0.6 1.7 No good — Comp. ex. 8 9 A coil 2.1 No. 5 — 1.6 0.9 3.8 Good 9 Example 1 10 A coil 2.0 No. 5 — 1.4 0.8 2.2 Good 8 Example 2 11 A coil 2.0 No. 5 500° C. × 1 h 1.3 0.8 2.9 Good 7 Example 3 12 A coil 1.9 No. 5 500° C. × 1 h 1.5 1.0 3.4 Good 6 Example 4 13 A coil 2.0 No. 5 600° C. × 1 h 1.2 0.8 3.0 Good 2 Example 5 14 A coil 1.9 No. 5 600° C. × 1 h 1.3 0.9 3.7 Good 2 Example 6 15 B coil 1.9 No. 5 — 1.4 0.9 3.9 Good 5 Example 7 16 B coil 1.9 No. 5 — 1.6 0.9 3.5 Good 9 Example 8 17 B coil 1.9 No. 5 500° C. × 1 h 1.3 0.9 4.4 Good 3 Example 9 18 B coil 1.9 No. 5 600° C. × 1 h 1.2 1.0 4.6 Good 1 Example 10 19 G coil 1.9 No. 5 — 1.0 0.9 2.8 Good 20 Example 11 20 C plate 2.0 No. 13B — 1.4 0.8 2.6 Good — Example 12 21 D plate 2.0 No. 13B — 1.5 0.8 3.0 Good — Example 13 22 E plate 2.0 No. 13B — 1.2 0.7 2.2 Good — Example 14 23 F plate 2.0 No. 13B — 1.3 0.7 2.1 Good — Example 15 24 H plate 1.9 No. 13B — 1.3 0.8 2.3 Good — Example 16 25 I plate 2.0 No. 13B — 1.0 1.0 2.4 Good — Example 17 26 I plate 2.0 No. 13B — 1.1 0.8 3.3 Good — Example 18 27 J plate 2.4 No. 13B — 1.3 0.8 3.2 Good — Example 19 28 K plate 2.5 No. 13B — 1.2 0.8 2.8 Good — Example 20 29 L plate 2.5 No. 13B — 1.2 0.8 3.0 Good — Example 21 30 B coil 1.9 No. 5 — 1.2 0.8 2.0 Good 8 Example 22 31 B coil 1.9 No. 5 — 1.1 0.8 2.1 Good 7 Example 23 32 B coil 1.9 No. 5 550° C. × 1 h 1.2 0.8 2.5 Good 3 Example 24 33 B coil 1.9 No. 5 550° C. × 1 h 1.1 0.8 2.5 Good 2 Example 25

FIGS. 4 (a), (b) and (c) are magnified photographs showing respective shape of fractured portions of the tensile specimens of examples 1, 6 and 16 as representative examples of all. The fractured portions of steel plates according to these examples are characterized by large local construction (high reduction type of area) as compared with the fractured portions in the specimens of comparative examples 1 and 6 shown in FIGS. 5 (d) and (e), therefore it is seen that the steel plates of the inventive examples are excellent in the tough-ductility. These photographs were taken from the side of respective specimens as shown in FIG. 5 (f).

As shown in Table 3, the steel plates and the coils according to this invention which has SD values of 1.8×10² MPa or more, have high strength of 1 GPa or above in all cases, and they are high strength steel plates excellent in the tough-ductility. Additionally, the steel plates of examples 1 to 13, 16 to 25 have yield ratios of 0.8 or more, and they are high strength steel plates excellent in proof stress and also in tough-ductility. Furthermore, in the steel plate coils according to this invention, dispersion of the tensile strength is slight in the cross direction of the coil, the dispersion of the tensile strength becomes further low by subjecting the coil to tempering, and it is confirmed that stability of the properties is improved by the tempering.

INDUSTRIAL APPLICABILITY

The steel plate according to the present invention is possible to be widely applied as a steel plate for parts of transportation equipments such as motor cars, air crafts, shipping or so, and construction materials. 

1. A steel plate characterized in that tensile strength is not lower than 1 GPa at the normal temperature, and stress decrement (SD) after uniform elongation in a stress-strain diagram obtained by a tensile test using a tabular specimen is not lower than 1.8×10² MPa.
 2. The steel plate as set forth in claim 1, yield ratio (YR) of which is not lower than 0.7.
 3. The steel plate as set forth in claim 1, yield ratio (YR) of which is not lower than 0.8.
 4. The steel plate as set forth in claim 1, matrix structure of which is composed of at least one of martensite and bainite, and does not substantially contain proeutectoid ferrite.
 5. A steel plate coil made by winding up the steel plate as set forth in claim
 1. 6.-11. (canceled)
 12. The steel plate as set forth in claim 2, the matrix structure of which is composed of at least one of martensite and bainite, and does not substantially contain proeutectoid ferrite.
 13. The steel plate as set forth in claim 3, the matrix structure of which is composed of at least one of martensite and bainite, and does not substantially contain proeutectoid ferrite.
 14. A steel plate coil made by winding up the steel plate as set forth in claim
 2. 15. A steel plate coil made by winding up the steel plate as set forth in claim
 3. 16. A steel plate coil made by winding up the steel plate as set forth in claim
 4. 17. A steel plate coil made by winding up the steel plate as set forth in claim
 12. 18. A steel plate coil made by winding up the steel plate as set forth in claim
 13. 